Individual loudness sensation is very important when fitting or adapting hearing aids. Hearing loss is typically described by the audiogram and includes the levels of the just perceivable sounds (hearing threshold level, HTL), between typically 125 Hz and 8 KHz and the levels of the sounds that lead to an uncomfortably high loudness sensation (uncomfortable loudness level, UCL). Low-level sounds have to be amplified for the hearing-impaired person so that the same become audible again. However, hearing-impaired persons typically have similar levels for the uncomfortable loudness level as persons with normal hearing. This phenomenon is called recruitment. This means that the amplification has to be significantly reduced for high-level signals in order to make the same not “too loud” for the hearing-impaired person. In current hearing aids, this level and frequency-dependent amplification is performed by multi-band dynamic compressors. The same split the input signal in different frequency bands, measure the current level in each frequency band and in that way the same can calculate and apply the desired amplification. One goal when adapting the amplification values to the individual hearing is the substantial normalization of loudness perception. However, narrowband loudness compensation in each frequency channel frequently results in a loudness in broadband and broadband binaural signals that is perceived as being too high. Thus, there is the technical problem that no loudness compensation can be obtained for narrow and broadband signals with a multiband dynamic compressor having independent amplification regulation in the frequency bands, since a further distinction of the signal type has to be made for applying the correct amplification values. Different opinions exist on the relevance of this problem, which are supported by different studies. However, there are significant empirical indications that current hearing aid adjustments in day-to-day hearing situations are perceived as too loud at high ambient levels. It is the object of the present invention to solve this specific problem in a practical manner.
Today, fitting of a multiband dynamic compressor to the individual hearing loss is typically performed based on a prescriptive fitting formula, which includes the audiogram as input parameter. Loudness-based methods increase the measurement effort for the initial fitting, since loudness scaling has to be performed in addition to the audiogram. So far, loudness-based methods could not establish themselves with respect to threshold-based descriptive fittings in daily clinical practice, since the measurable advantage in fitting is only limited and hence no time is saved during fine tuning. The current standard for fitting hearing aids are audiogram-based regulations such as NAL-NL2, DSL [i/o] or manufacturer-specific fitting rules, such as ConexxFit by Siemens. These threshold-based fitting rules are typically configured for optimizing speech comprehensibility and recovery of loudness of broadband signals (NAL-NL1 speech signal) and additionally include empirical correction factors (NAL-NL2 amplification reduction, since NAL-NL1 tended to be evaluated as being too loud). In loudness-compensating methods, an attempt has been made to recover the narrowband loudness functions or to normalize loudness of speech. After the initial fitting, fine-tuning is performed by the hearing aid audiologist in order to adjust the device to the individual loudness sensation and the subjective preference, typically for broadband signals. A possible algorithm using the bandwidth and the level of a signal, respectively, for applying different amplification values for narrowband and broadband signals is described in [2]. The question remains how an algorithm can be adjusted for different signal types (e.g. narrowband and broadband). A possible solution is the usage of loudness models for signal-dependent regulation of the amplification in multiband dynamic compressors, such as described in [2]. A disadvantage when using a generalized loudness model are estimation errors in individual loudness evaluation of narrowband and broadband signals that are considerable, in particular in the aided condition.
It is a specific disadvantage of hearing aid adjustments that the same are not suitable for all situations. One reason for this is that adjustments are typically not made for monaural signals.
In other words, this means that a dynamic compressor within a hearing aid for an individual who will use the hearing aid will be adjusted first, e.g., for the left ear and then for the right ear. Even when the adjustments are made such that the same fit equally for narrowband and broadband signals, it has become clear that these adjustments are still too loud, e.g. for binaural signal types. If, however, the amplification values are reduced in order to obtain, if possible, comfortable loudness for such signals, other signals would again be too low.
This causes the problem that hearing aid adjustment works relatively well for specific signal types but is not suitable for other signal types which is due to the non-linearity of human hearing on the one hand and due to the many different types of hearing impairments on the other hand.